A photovoltaic device, or solar cell, is device that converts the energy of light directly into electricity by the photovoltaic effect. A conventional photovoltaic system therefore comprises an array of photovoltaic modules, with each photovoltaic module comprising multiple photovoltaic devices integrated together and encapsulated to provide protection from both mechanical damage and the weather. FIG. 1 illustrates schematically (i.e. is a simplified representation that is not to scale) an example of a photovoltaic array comprising multiple photovoltaic modules, with each photovoltaic module comprising multiple photovoltaic devices.
FIG. 2 illustrates schematically an exploded view of an example of conventional photovoltaic module comprising multiple silicon solar cells. In such a conventional photovoltaic module, multiple silicon solar cells are sandwiched between an insulating back sheet and a transparent front sheet using adhesive encapsulant materials, such as poly ethylene-vinyl acetate (EVA) or polyvinyl butyral (or PVB), with an edge sealant (e.g. polyisobutylene, silicones and epoxy/acrylic resins) deposited around the periphery to provide some protection against the ingress of moisture. The components of such a conventional photovoltaic module are secured together in a lamination process that involves stacking the components together, applying a vacuum to remove any air and other volatiles, the application of pressure to ensure a good surface contact and adhesion between the different layers, and heating the components, typically to temperatures of around 160° C. for 15 to 30 minutes, to melt the layers of encapsulant material and the edge sealant to secure everything in place. To complete the module, a metal frame is then typically assembled around the edges of the module to increase mechanical stability and to facilitate mounting of the module, and a junction box is attached to an external surface of the module (typically the external face of the back sheet) that provides an electrical connection through the laminated structure with the photovoltaic devices contained within the module.
Whilst the encapsulation methods and materials used for conventional first generation photovoltaic modules is intended to provide some protection against the ingress of moisture this is largely in order to prevent the corrosion of the electrical connectors (e.g. conductive traces/wires/ribbons) that string together the photovoltaic devices within the module and to prevent moisture induced delamination, as silicon-based photovoltaic devices are not particularly sensitive to moisture. Consequently, the use of top and bottom layers of encapsulant material together with an edge sealant has been accepted as sufficient for conventional photovoltaic modules. In contrast, some of the second and third generation photovoltaic materials are highly sensitive to the effects of moisture, such that even the very low moisture-permeability and diffusivity sealant materials used in conventional photovoltaic modules do not provide sufficient protection against moisture to prevent degradation of these photovoltaic materials within the required lifetime of a module.
For example, one class of photovoltaic materials that has attracted significant interest has been the perovskites. Materials of this type form an ABX3 crystal structure which has been found to show a favourable band gap, a high absorption coefficient and long diffusion lengths, making such compounds ideal as an absorber in photovoltaic devices. Early examples of the use of perovskite materials in photovoltaic application are reported by Kojima, A. et al., 2009. Organometal halide perovskites as visible-light sensitizers for photovoltaic cells. Journal of the American Chemical Society, 131(17), pp. 6050-1 in which hybrid organic-inorganic metal halide perovskites were used as the sensitizer in liquid electrolyte based photoelectrochemical cells. Kojima et al report that a highest obtained solar energy conversion efficiency (or power energy conversion efficiency, PCE) of 3.8%, although in this system the perovskite absorbers decayed rapidly and the cells dropped in performance after only 10 minutes.
Subsequently, Lee, M. M. et al., 2012. Efficient hybrid solar cells based on meso-superstructured organometal halide perovskites. Science (New York, N.Y.), 338(6107), pp. 643-7 reported a “meso-superstructured solar cell” in which the liquid electrolyte was replaced with a solid-state hole conductor (or hole-transporting material, HTM), spiro-MeOTAD. Lee et al reported a significant increase in the conversion efficiency achieved, whilst also achieving greatly improved cell stability as a result of avoiding the use of a liquid solvent. In the examples described, CH3NH3Pbl3 perovskite nanoparticles assume the role of the sensitizer within the photovoltaic cell, injecting electrons into a mesoscopic TiO2 scaffold and holes into the solid-state HTM. Both the TiO2 and the HTM act as selective contacts through which the charge carriers produced by photoexcitation of the perovskite nanoparticles are extracted.
Further work described in WO2013/171517 disclosed how the use of mixed-anion perovskites as a sensitizer/absorber in photovoltaic devices, instead of single-anion perovskites, results in more stable and highly efficient photovoltaic devices. In particular, this document discloses that the superior stability of the mixed-anion perovskites is highlighted by the finding that the devices exhibit negligible colour bleaching during the device fabrication process, whilst also exhibiting full sun power conversion efficiency of over 10%. In comparison, equivalent single-anion perovskites are relatively unstable, with bleaching occurring quickly when fabricating films from the single halide perovskites in ambient conditions.
More recently, WO2014/045021 described planar heterojunction (PHJ) photovoltaic devices comprising a thin film of a photoactive perovskite absorber disposed between n-type (electron transporting) and p-type (hole transporting) layers. Unexpectedly it was found that good device efficiencies could be obtained by using a compact (i.e. without effective/open porosity) thin film of the photoactive perovskite, as opposed to the requirement of a mesoporous composite, demonstrating that perovskite absorbers can function at high efficiencies in simplified device architectures.
Whilst the perovskite materials show significant potential for providing solar energy at a much lower cost than traditional technologies, they have been shown to be extremely sensitive to moisture induced degradation, which impacts on both the short-term and long-term stability of photovoltaic devices that make use of photoactive perovskite materials.